In my U.S. Pat. No. 4,684,343 I describe a method of using a U-shaped, U-section, and imperforate tray having a buccal edge and an opposite lingual/palatal edge to take an impression of a human patient's jaw having a hard U-shaped dental arch, a mobile mucosa and a stationary mucosa to each side thereof, at least one tooth, and a linear action boundary between the two mucosae. According to this earlier invention one first selects from a set of anatomically differently dimensioned such trays the tray capable of fitting over the dental arch of the current patient with the buccal edge outside the arch and the lingual/palatal edge within the arch, with the tray generally uniformly spaced at most 5 mm from the arch and with the buccal and lingual/palatal edges engaging the mobile mucosa generally beyond the linear action boundary. This tray is then filled with a high-viscosity impression material and the filled and selected tray is pressed over said current patient's dental arch to force the buccal and lingual/palatal edges into tight engagement against the mobile mucosa past the linear action boundary with the tray generally uniformly spaced at most 5 mm from the arch to form between the tray and the dental arch a substantially closed and generally constant-section chamber hydraulically confining the impression material. The confined material is then compressed in the chamber against the dental arch by pressing the tray toward the arch while mobilizing the mobile mucosa to hydraulically press the confined material into form-fit engagement with the stationary mucosa and without substantial leakage of the impression material from the chamber.
This system has proven itself to be very good, but still has two modest drawbacks:
First of all it is impractical to provide a set of impression trays that is large enough to ensure a good fit with any mandible or maxilla. The variation in human anatomy is so very great that a very tight fit is only rarely possible. Thus there is inevitably some leakage from the chamber and, as a result, the highly viscous impression material is not pressurized sufficiently to conform to all of the contours of the region being worked on. Using a less viscous material that would more easily get into the various recesses and undercuts is impossible in working on a dentate jaw since it is necessary as described in my earlier patent to make the impression of the hard dental structures, with the softer ones pushed back or out of the way, and such less viscous material would be even more likely to leak out of the chamber which, as described above, is rarely sealed perfectly. Furthermore the less viscous material is more likely to leak out of the tray, so it will be pressurized even less than the high-viscosity material.
In addition once the impression is made it is quite difficult to separate it from the jaw in question. Since the material gets into recesses and undercuts, and also since the high-viscosity impression material hardens to a fairly stiff body, pulling it off entails deforming it and subjecting the contacting jaw structures to substantial stress. Once again using a more easily deformed softer impression material would avoid this problem, but would not allow one to proceed hydraulically as described above.
Accordingly it is known to make a first impression using one of a standardized set of trays and a low-viscosity impression material. From this rough impression a rough positive model is made and a custom acrylic tray is then made that provides the desired 1 mm to 5 mm spacing. This custom tray is therefore a very good fit and it is subsequently used with a high-viscosity material in a pressurized hydraulic procedure to form a very accurate impression from which another model is made, and this second model is used to construct the inlay, crown, bridge, partial denture, or the like. Such a procedure produces excellent results but has the first disadvantage that it requires two different impressions and an extra positive model to be made, greatly increasing the difficulty and cost of the procedure, and, second, the high-viscosity material is still hard to strip off the jaw.